‘Hot cloud’ theory of Moon formation gets boost

The Earth and Moon share a past etched in silicon, and it hints that the favoured theory of how the Moon formed isn’t quite right.

Most astronomers believe that the Moon formed when a Mars-sized object struck Earth. If this is the case, the Moon should contain mainly rock from the impacting body. However, Alex Halliday of the University of Oxford and his colleagues have evidence suggesting that this isn’t so.

The team studied the ratios of light to heavy isotopes of various elements in rock from the Earth, Moon and meteorites. For most elements, the composition was the same, but samples from the Earth and Moon showed a puzzling preference for the heavy forms of iron and silicon. The Moon’s isotopic make-up was identical to Earth’s, with no trace of an impacting object (Nature, vol 447, p 1102).

Halliday thinks that the results support a modified giant impact theory proposed two years ago by Kaveh Pahlevan and Dave Stevenson of the California Institute of Technology, Pasadena. They suggested that the impact of a Mars-sized body first created a hot cloud of rock vapour. This mixed together as it cooled – obliterating any unique isotopic signatures of the impacting body – before forming the Moon.

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